The present invention generally relates to communications systems and, more particularly, to a receiver.
In the ATSC (Advanced Television Systems Committee) standard for digital terrestrial television (DTV) in the United States (e.g., see, United States Advanced Television Systems Committee, “ATSC Digital Television Standard”, Document A/53, Sep. 16, 1995), the modulation system consists of a suppressed carrier vestigial sideband (VSB) modulation with an added small in-phase pilot at the suppressed carrier frequency, 11.3 dB below the average signal power. An illustrative spectrum for a ATSC VSB signal is shown in FIG. 1.
A typical ATSC-VSB receiver includes a carrier tracking loop (CTL) that processes a received ATSC VSB signal to both remove any frequency offsets between the local oscillator (LO) of the transmitter and LO of the receiver and to demodulate the received ATSC VSB signal down to baseband from an intermediate frequency (IF) or near baseband frequency (e.g., see, United States Advanced Television Systems Committee, “Guide to the Use of the ATSC Digital Television Standard”, Document A/54, Oct. 4, 1995; and U.S. Pat. No. 6,233,295 issued May 15, 2001 to Wang, entitled “Segment Sync Recovery Network for an HDTV Receiver”). The CTL generally includes: a Hilbert filter and corresponding delay, a complex multiplier, a phase detector, a first order loop filter, with a proportional plus integrator path, a numeric controlled oscillator (NCO) and a sine-cosine lookup table. Generally, the ATSC receiver must detect whether the CTL is “locked” or “unlocked” to the received ATSC VSB signal. For example, if the ATSC receiver detects that the CTL is locked, then the ATSC receiver determines that the received ATSC VSB signal is “good” and can be used for subsequent recovery of the data conveyed therein. However, if the ATSC receiver detects that the CTL is unlocked, then the ATSC receiver determines that the received ATSC signal is “bad” such that portions of the ATSC receiver may then be reset to, e.g., flush out any recovered data now associated with the bad received ATSC VSB signal, i.e., erroneous data. In addition, after the ATSC receiver detects that the CTL is locked, the CTL loop filter parameter may be changed to decrease the loop bandwidth and reject thermal noise.
Typically, the ATSC receiver determines whether the CTL is locked by using the loop filter integrator of the CTL. For example, a threshold value is established and if a signal from the loop filter integrator of the CTL (the “lock signal”) changes above the threshold value in a specified amount of time, the CTL is considered unlocked by the ATSC receiver. Unfortunately, the behavior of the loop filter integrator—and therefore the lock signal—is affected by impulse noise, thermal noise and loop bandwidth of the CTL. As a result, erroneous detections of locked and unlocked conditions may occur. For example, an unlocked condition may be falsely detected if the threshold is small compared with the noise power and loop bandwidth, or a locked condition may be falsely detected if the threshold is large compared with the noise power and loop bandwidth.